Re: [PATCH] tmpfs: use ida to get inode number

[Hugh Dickins <hughd@google.com>]

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On Thu, 21 Nov 2019, zhengbin (A) wrote:
> On 2019/11/20 23:45, Matthew Wilcox wrote:
> > On Wed, Nov 20, 2019 at 10:23:18PM +0800, zhengbin wrote:
> >> I have tried to change last_ino type to unsigned long, while this was
> >> rejected, see details on https://patchwork.kernel.org/patch/11023915.
> > Did you end up trying sbitmap?
> 
> Maybe sbitmap is not a good solution, max_inodes of tmpfs are controlled by mount options--nrinodes,
> 
> which can be modified by remountfs(bigger or smaller), as the comment of function sbitmap_resize says:
> 
>  * Doesn't reallocate anything. It's up to the caller to ensure that the new
>  * depth doesn't exceed the depth that the sb was initialized with.
> 
> We can modify this to meet the growing requirements, there will still be questions as follows:
> 
> 1. tmpfs is a ram filesystem, we need to allocate sbitmap memory for sbinfo->max_inodes(while this maybe huge)
> 
> 2.If remountfs changes  max_inode, we have to deal with it, while this may take a long time
> 
> (bigger: we need to free the old sbitmap memory, allocate new memory, copy the old sbitmap to new sbitmap
> 
> smaller: How do we deal with it?ie: we use sb->map[inode number/8] to find the sbitmap, we need to change the exist
> 
> inode numbers?while this maybe used by userspace application.)
> 
> >
> > What I think is fundamentally wrong with this patch is that you've found a
> > problem in get_next_ino() and decided to use a different scheme for this
> > one filesystem, leaving every other filesystem which uses get_next_ino()
> > facing the same problem.
> >
> > That could be acceptable if you explained why tmpfs is fundamentally
> > different from all the other filesystems that use get_next_ino(), but
> > you haven't (and I don't think there is such a difference.  eg pipes,
> > autofs and ipc mqueue could all have the same problem.
> 
> tmpfs is same with all the other filesystems that use get_next_ino(), but we need to solve this problem one by one.
> 
> If tmpfs is ok, we can modify the other filesystems too. Besides, I do not  recommend all file systems share the same
> 
> global variable, for performance impact consideration.
> 
> >
> > There are some other problems I noticed, but they're not worth bringing
> > up until this fundamental design choice is justified.
> Agree, thanks.

Just a rushed FYI without looking at your patch or comments.

Internally (in Google) we do rely on good tmpfs inode numbers more
than on those of other get_next_ino() filesystems, and carry a patch
to mm/shmem.c for it to use 64-bit inode numbers (and separate inode
number space for each superblock) - essentially,

	ino = sbinfo->next_ino++;
	/* Avoid 0 in the low 32 bits: might appear deleted */
	if (unlikely((unsigned int)ino == 0))
		ino = sbinfo->next_ino++;

Which I think would be faster, and need less memory, than IDA.
But whether that is of general interest, or of interest to you,
depends upon how prevalent 32-bit executables built without
__FILE_OFFSET_BITS=64 still are these days.

Hugh